A. Field of Invention
This invention pertains to the methods and apparatuses related to monitoring the condition of structures and structural joints and more specifically to methods and apparatus related to measurement devices for measuring strain and temperature of structural joints.
B. Description of the Related Art
In any structure—a civil, mechanical, aerospace, or armored structure—joints react to extreme loading conditions and are susceptible to failure when subjected to intense loads, severe environments, and/or extended usage. One method to determine the condition of structures and structural joints is through strain measurement. One method of measuring strain is using fiber Bragg gratings.
Strain sensing using fiber Bragg gratings is known in the art. U.S. Pat. No. 5,828,059 titled “Transverse Strain Measurements Using Fiber Optic Grating Based Sensors” and issued to E. Udd, discloses a “system and method to sense the application of transverse stress.” U.S. Pat. No. 5,591,965 titled “Multiparameter Sensor System Using a Multiple Grating Fiber Optic Birefringent Fiber” and issued to E. Udd, discloses a sensor “particularly useful for the measurement of three axes of strain and temperature at a single location in composite material.” In a previous design, as shown in U.S. Pat. No. 5,945,665, titled “Bolt, Stud or Fastener Having an Embedded Fiber Optic Bragg Grating Sensor for Sensing Tensioning Strain” and issued to Hay, the fiber sensor has been secured into a fastener without pre-stressing the sensor. Data shown in FIG. 17 shows the inherent shift in load when an un-stretched sensor is unloaded. This shift in the data is unacceptable as it would be construed as actual load shift in the structure.
Therefore, what is needed is a method and apparatus that can monitor joints periodically or continuously for their condition to provide early warning of failure. In addition, what is needed is a method and apparatus with the capability to provide consistent data without hysteresis when unloaded. The present invention overcomes one or more of the disadvantages of the prior art. Furthermore, the strain sensing capabilities of the present invention are enhanced using advanced fiber Bragg grating techniques.